The present invention relates to an automatic heating apparatus having a sensing system which senses steam of elevated temperature generated from a heated material in accordance with the state of heating, and which controls a heating source in response to a detection signal delivered from the sensing system.
Various types of heating apparatus incorporating a system for automatically sensing the finish of heating of a heated material have hitherto been put into practice. The sensing system of such conventional automatic heating apparatus uses most generally, as a sensing element, a humidity sensor which senses a change in the humidity. However, since the humidity sensor serves to sense a change in the electric resistance of the sensor caused by molecules of water adsorbed on the surface of the sensor a complicated structure and operation have been required such that the contamination on the surface of the element is burnt away periodically with the use of a heater in order to maintain a stable performance over a long time while avoiding any deterioration in the sensitivity or the like attributable to contamination of the surface of the sensor.
On the other hand, we have proposed in foreign applications with the right of priority claiming based on Japanese Patent Application No. 63-194063 a sensing system in which a high temperature gas or vapor such as steam gas generated from a heated material as heating progresses, is drawn out through a vent hole formed in the wall of a heating chamber and brought into contact with a pyroelectric element provided outside of the heating chamber so as to sense the finish of the heating in accordance with a voltage produced by the pyroelectric element In this type of sensing system, the sensing mechanism is based on a physical phenomenon of heat transfer from steam to the pyroelectric element. Thus, the sensitivity is not changed greatly due to the contamination of the surface of the element unlike the conventional humidity sensors, resulting in an advantage that it is theoretically possible to provide a very simple sensing system.
This system, however, has an inherent defect as well such that long-time cooking causes the temperature of the pyroelectric element itself to rise and, as a result, the difference in the temperature between the pyroelectric element and the steam generated from the food becomes small, resulting in a lowering of sensitivity. To cope with this, it is required to use the pyroelectric element while cooling it as suggested in the above-mentioned foreign applications, and, in addition, it is required to develop an efficient sensing system at low cost which is commercially available.